GMC 2005 T-Series Owner's Manual page 84

The passenger compartment air filter can be accessed
from the outside of the vehicle on the front passenger
side. It is concealed by the front access panel. Pull the
latch inside the cab on the driver's side of the vehicle
to open the front access panel.
To inspect and clean the passenger compartment air
filter, use the following steps:
3-20
1. Pull the tab located at
the bottom of the
screen.
3. Rinse both sides of the filter with clean water.
4. Reinstall the filter and screen after cleaning.
2. Pull the tab and
remove the filter.